If you have seen the film series "Meet the Parents," you will likely remember Robert De Niro’s character (a former CIA agent) putting his thumbs on Ben Stiller’s character’s wrists as a means for detecting lies. The rationale of this test relies on the fact that the heart rate rises in response to stress, and an elevated heart rate would indicate that a subject is lying. I have no idea whether this technique is accurate for detecting lies. I do know one thing for sure, though. The test does not identify lies but measures the physiological reaction that relates to a lie.
Most readers might wonder how this links to our laboratory forage analyses. Believe it or not, this analogy can help us understand the empirical methods of forage analysis.
In theoretical methods of analysis, a specific analyte is detected and measured. For example, calcium (the analyte) is identified and quantitatively measured in blood to diagnose clinical or subclinical hypocalcemia. Similarly, glucose (the analyte) is identified and quantitatively measured in blood to diagnose diabetes. In these two techniques, we know the analytes we are measuring.
In empirical methods of analysis, however, no specific analytes exist, and this is true for numerous forage analyses. For example, what is fiber? There is no such thing as "fiber." Fiber is not a specific analyte that can be detected and quantified. Instead, we use a technique that solubilizes most contents of plant cells in neutral detergent and separates the insoluble cell wall components. This insoluble residue is what we know as neutral detergent fiber or NDF.
Understanding empirical methods of analysis becomes more relevant when evaluating undegraded NDF or uNDF. By definition, uNDF is the remaining NDF or "fiber" after a 240-hour fermentation in rumen fluid (either in vitro or in situ), and it reflects the fraction of the NDF that does not provide any energy to the animal. Based on this definition, uNDF has become an indicator of forage quality extensively used by dairy nutritionists. However, quantifying the concentration of uNDF in forages is complicated for at least two reasons.
First, the determination of uNDF is an empirical method of analysis that does not measure a specific analyte. Second, and also related to the previous one, the determination of uNDF is subject to variations of the rumen environment among cows. To illustrate, imagine you need to select a forage based on two forage laboratory reports, reports A and B, that state 30% and 25.2% uNDF, respectively (both on a dry matter basis). I am pretty sure that most you would select the forage from report B, as it has less uNDF and provides more energy. With that being said, would you believe me if I tell you that these two values belong to the same forage (in this case, triticale silage)? The only difference is that uNDF was determined using two different cows.
In conclusion, understanding that most forage analysis techniques are empirical methods is very important, especially when major decisions are made based on these analyses.